The invention relates to a method and control system for controlling electric motors, especially motors carrying relatively strong current. Such motors are widely used for the operation of ship thrusters where it is also particularly important to assure reliable operation.
Normally, direct-current motors are used, and the current to both the motor rotor or armature and the field coils or field magnets is connected via an operating relay. The operation and the direction of rotation of the motor is controlled by means of a joystick or the like that is connected to a power source such as a battery and via the operating relay provides the desired power supply to the motor.
One problem that may arise is that the operating relay or the main relay of the motor becomes locked in the on position, usually as a consequence of low voltage and subsequent relay failure and/or strong current, but also for other reasons. The motor will then rotate uncontrollably until the main power switch of the operating system is switched off. This will easily take some second or minutes for an unskilled or inexperienced user or skipper. If the motor operates a thruster which pushes the boat to port or to starboard in narrow stretches or water, the consequence may be personal injury and material damage.
The object of the invention is to provide a method for controlling electric motors and a control system for electric motors which intend to eliminate the problems of the aforementioned type.